Snap-hooks are, as is well known, made up of a body or hook in the form of a "C" of which rear, straight, central portion is extended by two curved loops, one at the top and one at the bottom, the free ends of which are connected, to close the snap-hook, by a pivoting finger moving on one of the two ends.
It is necessary for it to be both sufficiently strong and as light as possible. Its strength guarantees the safety of the climber. Its longitudinal tensile strength (breaking load along the direction of the above-mentioned rear central portion) can reach about three tonnes, whilst its transverse tensile strength (breaking load of the ring formed by the snap-hook, in a direction perpendicular to the rear central portion) is about one tonne. This strength must be balanced against the lightness of the snap-hook so as not to over-weight the climber's equipment. Also, the ropes passed through the snap-hook must not, when under tension, be damaged by the snap-hook. It follows that the internal rim of the snap-hook must offer to the tautened ropes a semi-circular cross-section of adequate radius.
Attempts have been made to lighten the weight of the snap-hook in various ways. Whilst retaining a solid body, some snap-hooks have been made, not in steel, but in light aluminium alloys, some cast, some obtained by bending of circular section bar. On the other hand, some snap-hooks, in steel or other metals of comparable rigidity but of higher density than that of aluminium alloys, have been made from hollow tube. This is so in the case of snap-hooks whose external section is uniformly circular, described, for instance, in French Pat. Nos. 975.004, 70.30809 (2.060.668) and 77.04399 (2.342.423).